The instant invention relates to the method of making polysilazanes which can be thermally transformed into silicon nitride of exceptional purity in exceptionally high ceramic yield.
Difficulties inherent in traditional, powder-based techniques have fostered the investigation of chemical approaches to ceramic processing. These investigations led to the development of the "sol-gel" technique of ceramic fabrication, and the development of inorganic polymers which decompose into ceramics when heated, ("preceramic polymers"). While the sol-gel approach is most readily applied to the fabrication of oxide glasses and ceramics (e.g., silica, titania, cordierite, etc.), preceramic polymers are generally applied to the fabrication of non-oxide ceramics (e.g., silicon carbide, silicon nitride, boron nitride, etc.).
Advanced ceramics offer unique combinations of mechanical and electro-optical properties and are finding increased use in high technology applications. Current techniques of advanced ceramic component fabrication are based on pressureless green body consolidation in which voids often remain in the final microstructure. These voids have deleterious effects on the structural properties of the ceramic. Much of this residual porosity is generated when the organic binder used to consolidate the ceramic powder is burned off. One approach to minimizing void formations is through the substitution of the traditional fugitive binder with an organometallic polymer ("preceramic polymer") which will decompose into a predetermined ceramic when heated. The void space normally induced during organic binder burn-out would be replaced by the ceramic produced through the pyrolysis of the preceramic polymer.
Silicon nitride (Si.sub.3 N.sub.4) is a ceramic material in great demand for its superior high-temperature properties and excellent strength to weight ratio. Unfortunately, like other covalent materials, silicon nitride is difficult to sinter because of its low self-diffusivity. Densification can be achieved without applied pressure if sintering acids are used; however, the presence of these glassy materials in the ceramic seriously detracts from the material's high-temperature strength.
Although the utility of organometallics in the chemical vapor deposition (CVD) of ceramics was demonstrated over twenty-five years ago, many difficulties were encountered, and these efforts were abandoned. While efforts to develop useful preceramic polymers continued through the 1970's, it was not until the successful development of polymeric precursors to silicon carbide, and the commercial success of the Nicalon fibers made from them, that the investigation of polymeric precursors to silicon nitride and other advanced ceramics became widespread. Reviews of preceramic polymers in general and of polymeric precursors to silicon nitride (polysilazanes) have recently been published.
Synthesis of such polysilazanes has been accomplished by use of a two-stage dehydrocyclodimerization process as well as by the use of transition metal catalyzed reactions which require the use of high pressures and temperatures.
U.S. Pat. No. 4,460,638 also sets forth procedures for preparing silizane polymers which require either that a disilizane be used as one of the reactants or that the silane used must contain a vinyl group, a C.sub.1 to C.sub.3 alkyl radical, or a phenyl group. Also, the reaction temperature in the latter reaction must be at least 25.degree. C., with much higher temperatures being utilized.
The forgoing procedures for making the polysilazane are not entirely satisfactory in that they require complex reactants, high temperature and/or pressure reaction conditions, and/or result in polymeric precursors that do not produce silicon nitride of satisfactory yield or purity.